The power output of a turbojet engine is directly proportional to the turbine intake temperature. Presently, turbojet engines operate with pre-turbine temperatures approximating 1,800.degree. K. However, the specific fuel consumption of the turbojet engine, which is very high at those operational temperatures, decreases as the engine compression ratio increases. Thus, in order to maintain or increase the specific fuel consumption with the high turbine intake temperatures, it is necessary to raise the compression ratio of the engine.
These two parameters have necessitated the construction of double walled combustion chambers in order to provide the necessary thermal protection to the chamber and to increase their service life. A typical example of such double walled combustion chambers is shown in U.S. Pat. No. 4,704,874.
Cooling may be provided to the double walled combustion chambers by external convection by itself, or combined with a film cooling effect. The film cooling effect utilizes a film of air directed along the interior surface of the combustion chamber to avoid direct contact between the wall and the combustion chamber gases. The film is generated by secondary air intakes through the combustion chamber wall. Since the film cooling layer becomes increasingly diluted in the downstream direction, several such secondary air intakes must be arrayed along the length of the chamber to replenish the diluted film air.
Convection cooling may also be used between the two walls of the double walled conbustion chamber either by using air flowing in the same direction as the combustion gases, or in counterflow. In the case of counterflow convection cooling, the same stream of air may be used to form the peripheral cooling film on the inner wall of the combustion chamber after it has been used to cool the chamber by convection. However, such systems have typically required substantial air flow to achieve significant convection cooling.
The double walled construction has also presented problems, since the inner wall exposed to the combustion chamber gases (the hot wall) undergoes substantial expansion and contraction with respect to the outer or cold wall, which is not exposed to such high temperatures. Therefore, some means must be provided to attach the inner or hot wall to the outer or cold wall so as to allow relative movement due to the thermal expansion of the hot wall. U.S. Pat. No. 4,302,941 to Dubell discloses a combustion chamber structure in which an inner or hot wall is attached to an outer wall by means of bolts which provide relative radial movement between the inner and outer walls. Longitudinal strips attached to the outer surface of the inner or hot wall guide the convection air and serve as a limit to the radial movement of the inner wall with respect to the outer wall. However, while these strips are useful in directing the convection air, they generate substantial turbulence or wakes in the cooling film on the interior surface of the inner wall so as to reduce the effectiveness of this film. Also, the air dilution intake apertures being located directly at the film discharge also serve to degrade the effectiveness of the film.
Another form of double walled combustion chamber construction is shown in French Pat. No. 2,023,415. This structure utilizes staggered sleeves forming the inner, hot wall which are fixed in place at their upstream edge. The downstream edges of the sleeves have stops which limit the radial movement of the sleeves relative to an outer or cold wall. Ths combustion chamber construction utilizes same direction convection cooling with an internal cooling film, however, the film thickness is not entirely controlled in relation to the hot wall expansion. Also, the stops located on the downstream edges of the sleeves introduce turbulence into the cooling film so as to further degrade its uniformity and effectiveness.
French Pat. No. 2,422,035 discloses a combustion chamber construction in which the perturbations of the cooling film due to the dilution air intake orifices are limited by leaving a free space between the inner, hot sleeve and the tubular dilution orifices. This structure provides a downstream lip on the inner end of the dilution orifice tube in order to reestablish the cooling film that had been interrupted by the tube.